1. Field of the Invention
The present invention relates to integrated oxidation processes to efficiently reduce the sulfur and nitrogen content of hydrocarbons to produce fuels having reduced sulfur and nitrogen levels.
2. Description of Related Art
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from sulfur-containing sour crude oil pose health and environmental problems. The stringent reduced-sulfur specifications applicable to transportation and other fuel products have impacted the refining industry, and it is necessary for refiners to make capital investments to greatly reduce the sulfur content in gas oils to 10 parts per million by weight (ppmw), or less. In industrialized nations such as the United States, Japan and the countries of the European Union, refineries for transportation fuel have already been required to produce environmentally clean transportation fuels. For instance, in 2007 the United States Environmental Protection Agency required the sulfur content of highway diesel fuel to be reduced 97%, from 500 ppmw (low sulfur diesel) to 15 ppmw (ultra-low sulfur diesel). The European Union has enacted even more stringent standards, requiring diesel and gasoline fuels sold in 2009 to contain less than 10 ppmw of sulfur. Other countries are following in the direction of the United States and the European Union and are moving forward with regulations that will require refineries to produce transportation fuels with an ultra-low sulfur level.
To keep pace with recent trends toward production of ultra-low sulfur fuels, refiners must choose among the processes or crude oils that provide flexibility to ensure that future specifications are met with minimum additional capital investment, in many instances by utilizing existing equipment. Conventional technologies such as hydrocracking and two-stage hydrotreating offer solutions to refiners for the production of clean transportation fuels. These technologies are available and can be applied as new grassroots production facilities are constructed. However, many existing hydroprocessing facilities, such as those using relatively low pressure hydrotreaters were constructed before these more stringent sulfur reduction requirements were enacted and represent a substantial prior investment. It is very difficult to upgrade existing hydrotreating reactors in these facilities because of the comparatively more severe operational requirements (i.e., higher temperature and pressure conditions) to obtain clean fuel production. Available retrofitting options for refiners include elevation of the hydrogen partial pressure by increasing the recycle gas quality, utilization of more active catalyst compositions, installation of improved reactor components to enhance liquid-solid contact, the increase of reactor volume, and the increase of the feedstock quality.
There are many hydrotreating units installed worldwide producing transportation fuels containing 500-3000 ppmw sulfur. These units were designed for, and are being operated at, relatively mild conditions, i.e., low hydrogen partial pressures of 30 kilograms per square centimeter for straight run gas oils boiling in the range of 180° C.-370° C.
However, with the increasing prevalence of more stringent environmental sulfur specifications in transportation fuels mentioned above, the maximum allowable sulfur levels are being reduced to no greater than 15 ppmw, and in some cases no greater than 10 ppmw. This ultra-low level of sulfur in the end product typically requires either construction of new high pressure hydrotreating units, or a substantial retrofitting of existing facilities, e.g., by integrating new reactors, incorporating gas purification systems, reengineering the internal configuration and components of reactors, and/or deployment of more active catalyst compositions. Each of these options represents a substantial capital investment
Sulfur-containing compounds that are typically present in hydrocarbon fuels include aliphatic molecules such as sulfides, disulfides and mercaptans, as well as aromatic molecules such as thiophene, benzothiophene and its long chain alkylated derivatives, and dibenzothiophene and its alkyl derivatives such as 4,6-dimethyl-dibenzothiophene. Aromatic sulfur-containing molecules have a higher boiling point than aliphatic sulfur-containing molecules, and are consequently more abundant in higher boiling fractions.
In addition, certain fractions of gas oils possess different properties. The following table illustrates the properties of light and heavy gas oils derived from Arabian Light crude oil:
TABLE 1Feedstock NameLightHeavyAPI Gravity°37.530.5Carbonwt %85.9985.89Hydrogenwt %13.0712.62Sulfurwt %0.951.65Nitrogenppmw42225ASTM D86 DistillationIBP/5 V %° C.189/228147/24410/30 V %° C.232/258276/32150/70 V %° C.276/296349/37385/90 V %° C.319/330392/398  95 V %° C.347Sulfur SpeciationOrganosulfur Compoundsppmw45913923Boiling Below 310° C.Dibenzothiophenesppmw10412256C1-Dibenzothiophenesppmw14412239C2-Dibenzothiophenesppmw13252712C3-Dibenzothiophenesppmw11045370
Aliphatic sulfur-containing compounds are more easily desulfurized (labile) using conventional mild hydrodesulfurization methods, at mild operating conditions, i.e. hydrogen partial pressure of 10-30 kg/cm2, temperatures of 330-360° C., liquid hourly space velocity of 1-4 volume of liquid per volume of catalysts and per hour. However, certain highly branched aliphatic molecules can sterically hinder the sulfur atom removal and are moderately more difficult (refractory) to desulfurize using conventional hydrodesulfurization methods.
Among the sulfur-containing aromatic compounds, thiophenes and benzothiophenes are relatively easy to hydrodesulfurize. The addition of alkyl groups to the ring compounds increases the difficulty of hydrodesulfurization. Dibenzothiophenes resulting from addition of another aromatic ring to the benzothiophene family are even more difficult to desulfurize, and the difficulty varies greatly according to their alkyl substitution, with di-beta substitution being the most difficult to desulfurize, thus justifying their “refractory” appellation. These beta substituents hinder exposure of the heteroatom to the active site on the catalyst.
The economical removal of refractory sulfur-containing compounds is therefore exceedingly difficult to achieve, and accordingly removal of sulfur-containing compounds in hydrocarbon fuels to an ultra-low sulfur level is very costly utilizing current hydrotreating techniques. When previous regulations permitted sulfur levels up to 500 ppmw, there was little need or incentive to desulfurize beyond the capabilities of conventional hydrodesulfurization, and hence the refractory sulfur-containing compounds were not targeted. However, in order to meet the more stringent sulfur specifications, these refractory sulfur-containing compounds must be substantially removed from hydrocarbon fuels streams.
The development of non-catalytic processes for desulfurization of petroleum distillate feedstocks has been widely studied, and certain approaches are based on oxidation of sulfur-containing compounds described, e.g., in U.S. Pat. Nos. 5,910,440, 5,824,207, 5,753,102, 3,341,448 and 2,749,284. Well known oxidizing agents include gaseous forms of oxygen, such as air or pure oxygen. In addition, it is known to use aqueous oxidant such as hydrogen peroxide, or organic peroxides, as oxidizing agents.
Organic peroxides are a very versatile source of active oxygen atoms and radicals. Radicals are formed after the thermally induced homolysis of the peroxide bond. The major radical-molecule reactions are additions and homolytic bimolecular substitution reaction, e.g. H-abstraction, atom transfer, unimolecular reactions, e.g. decarboxylation, β-scission and rearrangements, e.g. 1,5-H-abstraction. In synthesis reactions, undesired radical-radical reactions such as radical combination and disproportionation can be avoided by proper choice of the type of peroxide and reaction conditions. Another major application of organic peroxides in synthesis is oxidation, which is a non-radical reaction.
There are several important parameters for the selection of peroxide for use in chemical reactions. Physical and chemical stability impacts the storage and handling properties, and the temperature dependent rate of decomposition determines the reactivity at the process conditions. Decomposition products of the peroxides, therefore, must be taken in account during the purification process.
Organic peroxides are well established synthetic agents in the manufacture of many pharmaceutical intermediates, herbicides, insecticides and various other fine chemicals. Organic peroxides offer opportunities to reduce the number of reaction steps in synthetic routes applying classical synthetic procedures.
Organic peroxides combine a number of interesting features for their application in organic synthesis, including high purity, high efficiency, favorable solubility in most organic systems thereby enabling homogeneous reaction conditions, well defined and temperature controlled reactivity, and favorable cost-to-performance ratios.
Organic peroxides can have a variety of characteristics depending on their chemical structure and reactivity. Reactivity of the peroxides depends on the peroxide group configuration and on the type of substituent. Organic peroxides can be classified into different groups depending on their chemical structures, as shown in Table 2:
TABLE 2Type of PeroxideStructureHydroperoxide Ketone peroxide Peroxyacid DialkylperoxideR1—O—O—R2 Peroxyester Peroxycarbonate Diacylperoxide Peroxydicarbonate Cyclic ketone peroxide
Thermally induced homolysis of the peroxide bonds yield oxy-radicals. The decomposition rate of the peroxides not only depends on the class of peroxide, but also on the type of R-group. Therefore, the reactivity and sensitivity of the peroxides to radical attack, i.e., induced decomposition, is strongly dependent upon its structure.
The thermal decomposition of organic peroxides is a first order reaction. Increase in temperature of about 10° C. results in a 2-3 fold increase in decomposition rate. The decomposition is further accelerated with catalysts possessing high oxidation potential. The half-life times of various organic peroxides varies from 0.1 to 10 hours on a range of temperature from 70° C. to 210° C.
Organic peroxides are commonly associated with safety hazards during experimental preparation and synthesis, storage and transportation. Thus, heightened safety precautions and measures are required when handling organic peroxides. Known safety measures include, cooling the organic peroxides to low temperatures, preparing the organic peroxides in a diluted medium such as water, and/or incorporation of chemical stabilizers. These precautions are necessary to minimize the likelihood of impact shock and thermal influence caused by exothermic reactions.
It would be desirable to provide an oxidative process for converting heteroatoms into their corresponding oxidation products that minimizes the need for safety measures required for handling of organic peroxides.